Integrated circuits are formed on wafers by well-known processes and materials. These processes typically include the deposition of thin film layers by sputtering, metal-organic decomposition, chemical vapor deposition, plasma vapor deposition, and other techniques. These layers are processed by a variety of well-known etching technologies and subsequent deposition steps to provide a completed integrated circuit.
A crucial component of integrated circuits is the wiring or metallization layer that interconnects the individual circuits. Conventional metal deposition techniques include physical vapor deposition (e.g., sputtering and evaporation) and chemical vapor deposition techniques. Integrated circuit manufacturers have also developed electrolytic and electroless plating techniques to deposit primary conductor films on semiconductor substrates.
Wiring layers traditionally contained aluminum and a plurality of other metal layers that are compatible with aluminum. Around 1997, new technology facilitated a transition from aluminum to copper wiring layers. This technology demanded corresponding changes in process architecture towards damascene and dual damascene architecture, as well as new process technologies.
Copper damascene and dual damascene circuits are produced by initially forming trenches and other embedded features in a wafer, as needed for circuit architecture. These trenches and embedded features are formed by conventional photolithographic processes in a nonconductive substrate, such as a silicon oxide. Usually, a barrier layer, for example, of silicon nitride or tantalum, is deposited next. An initial seed or strike layer, (e.g., a copper or ruthenium layer having a thickness of about 5 nanometers (nm) to 200 nm) is then deposited by a conventional physical or vapor deposition technique. The seed layer is used as a base layer to conduct current for electroplating thicker films. Thinner seed layers are preferred to reduce overhang and closure of very small features by metal from the seed layer. The seed layer functions as the initial cathode of an electroplating cell. Electrical contacts to the wafer are normally made at its edge.
In electroplating processes, it is generally desirable to control the thickness profile of the deposited metal to be as uniform as possible. A uniform profile is advantageous in subsequent etchback or polish removal steps, as well as in uniform void-free filling of vias and trenches. Prior art electroplating techniques are susceptible to thickness irregularities. Factors contributing to these irregularities include the size and shape of the electroplating cell, electrolyte depletion effects, hot edge effects, and the terminal effect.
A conventional electroplating bath typically contains the metal to be plated together with associated anions in an acidic solution. Copper electroplating is usually performed using CuSO4 and a chloride dissolved in an aqueous solution of sulfuric acid. In DC electroplating, additives such as accelerators, suppressors, and levelers are typically included in the electrolytic plating solution to improve electroplating behavior by, among others, enhancing chemical reactions, improving surface deposition, improving thickness uniformity, and enhancing filling of high aspect ratio features. Sulfuric acid provides high conductivity to the electrolyte, and chloride ions enhance additive performance.
Three types of electroplating bath additives are in common use, subject to design choice by those skilled in the art. A suppressor additive is used to decrease the current density, and thus the deposition rate on the surface of the wafer at a given applied voltage. This allows differentiation in deposition rate between the wafer surface and the inside of high aspect ratio features, thereby enhancing the void-free fill of high aspect ratio features. Typical suppressors are large molecules, typically having an average molecular weight (MW) in a range of about from 2,000 to 6,000 that increase the surface polarization layer and prevent copper ion from readily adsorbing onto the surface. Thus, suppressors function as blockers. Suppressors cause the resistance of the surface to be very high in relation to the electroplating bath. Trace levels of chloride or other ion may be required for suppressors to be effective.
Accelerator additives accumulate within the high aspect ratio features to increase the local current density relative to the suppressed field and thus aid in void free filling. Accelerator additives are normally catalysts that accelerate the plating reaction. Accelerators typically are rather small molecules (e.g., 300 MW) that normally contain sulfur, and they need not be ionic. Accelerators adsorb onto the surface and increase the flow of current. Accelerators may occur not as the species directly added to the electroplating bath, but as breakdown products of such molecules. In either case, the net effect of accelerators is to increase current flow and accelerate the reaction when such species are present or become present through chemical breakdown.
A leveler additive is present to improve overall deposit planarity and increase the ease of subsequent CMP processing. Levelers behave like suppressors, but tend to be more electrochemically active (i.e., are more easily electrochemically transformed) than suppressors. Levelers are typically consumed during electroplating. Levelers tend to supress plating on raised regions of the surface undergoing plating, thus, tending to level the plated surface.
In conventional electroplating solutions, the additive components are designed to provide ideal characteristics for bottom-up fill of trenches and vias, as well as planarization of plating above filled features. Organic additives are present in an electroplating solution in low concentrations, usually measured in parts per million (ppm). The low concentrations of organic additives must be closely controlled to achieve desired deposition behavior and desired properties of deposited metal.
Cyclic voltammetric stripping (CVS) is a widely used method for monitoring and controlling plating bath additives. CVS involves cycling the potential of an inert electrode (e.g., Pt) in a cleaning solution between fixed potential limits so that metal is ultimately plated onto the electrode and then stripped from the electrode surface. The cycling of potential is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface is avoided by periodically cycling the potential of the electrode in a plating solution without organic additives and, if necessary, polishing the electrode. Cyclic pulse voltammetric stripping (CPVS), also known as cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode to improve measurement precision. A rotating disk electrode is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions. A limitation of techniques using a rotating disk electrode is that the solution studied cannot be changed while a measurement is being completed.
In CVS and CPVS analyses, a metal deposition rate is determined from the current or charge passed during metal electrodeposition. Typically, the charge associated with anodic stripping of the metal from the electrode is measured. Various techniques involving cyclic voltammetry are described in U.S. Pat. No. 7,186,326, issued Mar. 6, 2007, and U.S. Pat. No. 7,270,733, issued Sep. 18, 2007, which are hereby incorporated by reference. Other less common techniques for monitoring the concentration of organic additives in electroplating solutions that do not involve CVS are described briefly in U.S. Pat. No. 7,270,733.
Currently used techniques for monitoring the concentration of organic additives are typically characterized by one or more of the following shortcomings. A common problem is the large size and large footprint associated with detection and analysis equipment. Another problem is the large amount of reagents used in analysis, and the corresponding high expense. Additionally, some techniques have demanding equipment requirements; for example, some techniques use expensive and unreliable syringe pumps, while others use complex frequency response locks in amplifiers and power supplies. Some techniques are suitable for measuring one type of organic additives, but are less suitable or unsuitable for measuring other types of organic additives. Further, commonly used techniques generally require complicated calibration and analysis. More importantly, the accuracy, precision and reliability of the measurements of some commonly used monitoring devices are often unsatisfactory.
Therefore, there is a need, particularly in the field of integrated circuit fabrication, for systems and methods useful for monitoring and controlling the concentrations of organic additives in electroplating solutions.